1. Field of the Invention
An exemplary aspect of the invention relates to a phase shift mask and a method for manufacturing the same, and a method for manufacturing an integrated circuit.
2. Description of the Related Art
For devices typified by semiconductor integrated circuits (hereinafter referred to as LSIs), particularly memory devices such as DRAMs, in which high-density transistors are arranged, the size of transistors has been reduced in recent years at an accelerated rate in order to increase the integration degree of the devices. For semiconductor process techniques, it is important to improve the transfer fidelity of a lithography technique to transfer an LSI pattern drawn on a photo mask onto a wafer in order to form the LSI pattern on a multilayer film stacked on the wafer.
The resolution R of an exposure apparatus, indicating the transfer fidelity of the lithography technique, is defined as R=k1×λ/NA, using three parameters, the exposure wavelength λ of the exposure apparatus, the numerical aperture NA of the projection lens, and the process constant k1. In general, in order to reduce the resolution R, it is desirable to reduce the exposure wavelength λ or to increase the numerical aperture NA of the projection lens. However, in any case, the exposure apparatus needs to be drastically modified. Obtaining an exposure apparatus with a higher resolution is not easy. Thus, techniques are available which improve the resolution by reducing only the process constant k1 of the exposure apparatus with λ and NA unchanged. The techniques are commonly called super-resolution techniques.
A phase shift mask technique, which is one of the super-resolution techniques, in which the phase of a portion of light passing through a photo mask which is transmitted through a particular pattern is shifted by 180° from the phase of a portion of the light which is transmitted through another pattern, can sharply increase the separative resolution between the two patterns to be sharply increased. The phase shift mask technique is known to be capable of reducing k1 more sharply than any other super-resolution techniques. Specifically, openings and phase shifters are alternately formed in a mask so that each opening is sandwiched between the phase shifters and each of the phase shifters allows the phase of light transmitted through the adjacent opening to be inverted. The common phase shifter is of an engraved type formed by engraving the phase shifter on the substrate. The engraved phase shifter has a thickness of λ/(2(n−1)) in an engraved portion wherein λ is the wavelength of irradiation light; the engraved phase shifter utilizes the property of light that while light is passing through a medium with a refractive index (n, where n>1), the wavelength of the light is 1/n of that of the light transmitted through the air, thereby providing a light path difference. Characteristically, the light intensity is equal to the square of a phase difference, and the separative resolution of light is highest at a phase inversion boundary portion.
However, in the corners of the bottom of the engraved phase shifter, transmitted light tends to be refracted and scattered outward. Thus, in the region where the phase shifter is formed, only the transmitted light except for scattered components contributes to the resolution of the pattern. That is, as shown in FIGS. 42 and 43 in Japanese Patent Application Laid-Open No. 2005-129805, the intensity of light transmitted through the region in which the phase shifter is formed is lower than that of light transmitted through the region in which the phase shifter is not formed. The imbalance of the light intensity may result in a dimensional difference in transfer patterns.
As a method for eliminating the imbalance of the light intensity, the following have been proposed:
(1) a method in which a shielding film extends like an eave at each opening in a phase shifter (see FIGS. 45 and 46 in Japanese Patent Application Laid-Open No. 2005-129805);
(2) a method of stacking two types of masks such that regions in which the phase shifter is formed in one mask are identical to regions in which the phase shifter is not formed in the other mask and regions in which the phase shifter is not formed in one mask are identical to regions in which the phase shifter is formed in the other mask (see FIGS. 1 to 3 in Japanese Patent Application Laid-Open No. 2005-129805);(3) a method of increasing the width of each opening in a phase shifter to compensate for the light intensity corresponding to scattered components (see FIG. 7 in Japanese Patent Application Laid-Open No. 2007-298546);(4) a method of increasing the width of each opening in a phase shifter and setting an inclination angle α for side walls of the phase shifter (see FIG. 1 in Japanese Patent Application Laid-Open No. 2007-298546); and(5) a method of forming a projecting portion in which a shielding film projects at each opening in a phase shifter and forming, on a substrate, a support portion configured to support the projecting portion (see FIGS. 1 and 2 in Japanese Patent Application Laid-Open No. 2005-321641).
An exemplary aspect of the invention provides a phase shift mask with a new structure.